1. Field of the Invention
The present invention relates to the manufacture of shaped articles made of an oxide-dispersion-strengthened alloy, preferably by hot forming plates or sheets made of such alloys.
2. Discussion of the Background
Oxide-dispersion-strengthened alloys are superalloys, especially nickel- and/or iron-based superalloys, containing as a dispersion within their matrix fine oxide particles which, by preventing dislocations from migrating, make the material very strong, this being up to very high temperatures.
Their very good tensile strength, thermal fatigue strength and corrosion resistance make these materials very advantageous for industrial applications in which components are highly stressed at high temperature, such as compressors, turbocompressors, aeronautical or aerospace turbines, and tools which can be used in the production and/or conversion of glass. These materials are obtained by powder metallurgy, by making a mechanical alloy of various metals with atomization of metal powders in a suitable mixer, which alloy is then forged, extruded and hot rolled At this stage, the material has undergone so-called "primary" recrystallization and the matrix of the alloy has a microstructure consisting of submicron ultrafine grains (the size is about 50 nm to 1 .mu.m) in which the small oxide particles are dispersed.
Improved strength properties may be obtained by a so-called "secondary recrystallization" heat treatment. This treatment, which consists of annealing at a defined temperature which depends solely on the composition of the alloy, is intended to increase the size of the grains. The treatment is considered to have succeeded when a so-called "abnormal" grain growth is observed which gives a final structure consisting of very coarse anisotropic grains of elongate shape, having a length of a few centimeters, slip at grain boundaries being very difficult in such a structure.
Unfortunately, it has been found that the success of such a treatment for one alloy specimen is not guaranteed for another one of identical composition and it has been shown that the technical history of a specimen has an influence on its ability to produce the desired abnormal growth; that is to say that the treatments, especially the heat and mechanical treatments, received by the alloy in order to form the specimen which it is intended to undergo to the secondary recrystallization annealing determine the behavior of the specimen during this treatment.
These differences in behavior have been demonstrated, for example, in EP-A-0,447,858 which proposes, as a consequence, a method for adapting the rate of temperature rise of the recrystallization treatment to a given substrate, such as a plate or a bar.
Among the important steps in the technical history of such an alloy may first of all be included the production of the alloy by mechanical means which involves extremely complex mechanisms and whose control is very difficult, and then the steps of consolidating and of forming the alloy to its final shape before recrystallization. Each of these steps contributes significantly to varying the ease with which the alloy can recrystallize, and a very slight fluctuation of one single treatment parameter is liable to affect the abnormal growth capability of the alloy very greatly.
In fact, the manufacturers of oxide-dispersion-strengthened alloys are able for the moment to control this capability roughly, but only for relatively simple shapes, and the only articles having a coarse-grained matrix which are available on the market are in the form of plates or of bars of round or rectangular cross-section.
On the other hand, the production of components with a more complex shape having a coarse-grained strengthened matrix poses a problem for which a fully satisfactory solution has not yet been found.
Attempts have already been made to manufacture shaped components from non-recrystallized materials, initially in the form of plates, or bars which are not cold-bendable, using hot-forming techniques (at around 1000.degree. C.), these components being subjected, once the forming has been completed, to recrystallization annealing intended to cause grain growth. However, although the alloy in its initial form does have, a priori, the capability of abnormal growth on recrystallization under conditions specified by the manufacturer, the shaped component can be recrystallized with greater or lesser ease and it frequently happens that, at the end of the process, the alloy does not develop a coarse-grained structure or develops it only over part of the cross-section of the component. These manufacturing runs generally include a proportion of defective articles which is much too high to be acceptable.
For the manufacture of objects having a symmetry of revolution, techniques have been proposed for manufacturing from bars of alloy, in which techniques the bars are converted into profiled elements, especially by circular hot rolling or by hot spin forming, and then subjected to recrystallization annealing. The main drawback of these techniques is the lack of precise control of the temperature of the component during its shaping, and consequently of the general conditions of the thermomechanical treatment. In fact, components manufactured in this way quite rarely have the desired microstructure at the point where it is necessary.
Another, simpler method has been proposed, in document EP-A-0,668,122, for manufacturing concave components forming container elements for a fluid, this time by stamping, starting with blank made of refractory alloy such as an oxide-dispersion-strengthened alloy. The process consists in coating the blank with a ceramic-based coating, then in carrying out hot-stamping forming operations during which the coating acts both as a lubricant and as a thermal insulant, and finally in subjecting the cooled component to a secondary recrystallization heat treatment. Although this technique allows the forming temperature to be well controlled, by virtue of the ceramic coating, it still happens that certain components do not develop the desired coarse-grained structure, or only do so over certain parts of their cross-section, even if one tries to modify the stamping temperature or the recrystallization temperature.
This is because, unlike the bars, the plates and sheets from which the blanks are cut out have a technical history involving many thermomechanical treatment operations, in particular hot rolling, which means that, although the capability of abnormal growth during secondary recrystallization is quite homogeneous within the same batch of plates, this capability sometimes radically changes from one batch to another.
Thus, although certain plates, after having been formed by stamping according to EPA-0,668,122, are capable of developing a coarse-grained microstructure typical of abnormal growth, it has been found that in other plates, of identical composition, the recrystallizability is inhibited during the process and the resulting components are defective. Of course, it would be desirable, in order to avoid too high a scrap rate, to find a way of converting these same plates to the desired shape without losing the capability of abnormal growth.